This invention relates to an electric power supply circuit for use in a digital carrier transmission system of a distant power feeding type to supply a load with a constant current and to a constant current unit for use in the electric power supply circuit.
A conventional electric power supply circuit of the type described comprises a plurality of constant current units connected in series to a load, such as a repeater amplifier and the like, included in a current path and has therefore a redundant structure in order to compensate for a failure of each constant current unit. In addition, it is a recent trend that the repeater amplifier has a wide variety of functions. Under the circumstances, the electric power supply circuit should produce an output current which amounts to 500 mA.about.2A. A maximum output voltage must reach 15 kV. Accordingly, each of the constant current units should have a high power and a high efficiency on DC-DC power conversion.
For this purpose, a constant current converter of a resonant type has been developed as each of the constant current units in order to accomplish an efficiency which is not lower than 90%.
As will later be described with reference to a few figures of the accompanying drawing, the constant current converter comprises an inverter circuit which is controllable by a local signal of a controllable frequency and which converts a d.c. input signal into an a.c. output signal. The a.c. output signal is sent through a resonant circuit to a rectifying and smoothing circuit to be converted into a resultant d.c. current. The resultant d.c. current is delivered to the load and is monitored by a current detector. A detection signal is supplied from the current detector through a control circuit to a voltage to frequency converter as a control voltage signal. The voltage to frequency converter supplies the inverter circuit with the local signal of the frequency controlled by the control voltage signal. With this structure, the resultant d.c. current can be kept substantially constant, as is well known in the art.
When a plurality of constant current converters are connected in series to the load, a resistor should be connected in parallel to each constant current converter so as to stably divide the load to the respective constant current converters. Otherwise, overload occurs in one or more of the constant current converters when the resultant d.c. current varies within a range determined for each of the constant current converters.
However, connection of the resistors to the converters results in degradation of a constant current characteristic and in a loss of electric power due to currents which flow through the respective resistors. Heat is inevitably produced in the resistors and requires a countermeasure. As a result, each of the converters becomes bulky.
In addition, the control circuit and the current detector should be installed in each of the converters, which makes the electric power supply circuit expensive. Furthermore, it takes a long time to adjust the whole current of the electric power supply circuit to a desired constant value.
A paper is contributed by Jun Chida et al to "National Convention Record (in English translation)" of the Institute of Electronics and Communication Engineers of Japan, published Mar. 5, 1984, pages 10-30, under the title of "Control Circuit for Series Resonant Constant Current Converter (in English translation)." In the Chida et al paper, an electric power supply circuit is disclosed which comprises a plurality of constant current converters connected in series to a load without any resistors so as to avoid the above-enumerated defects resulting from connection of the resistors. To this end, a set of current detectors is coupled to a set of control circuits outside of the converters to produce a control current signal common to the respective constant current converters. The control current signal is individually converted into control voltages of current to voltage converters installed in the respective constant current converters, respectively. The control voltages are further converted into local signals of frequencies controlled by the control voltages in the manner described before.
With this structure, each converter should have the current to voltage converter and therefore becomes intricate in structure.